Three endogenous mechanisms have emerged as potential candidates to acidify either whole stratum corneum (SC) or pH within more localized membrane domains: a) free fatty acid (FFA) generation from phospholipid (PL) hydrolysis, b) a non-energy-dependent sodium/proton membrane antiporter, NHE1, and c) urocanic acid (UCA) generation from histidine. By regulating pH, these mechanisms could regulate three key SC functions. The importance of pH for permeability barrier homeostasis is suggested by the delay in barrier recovery that occurs when perturbed skin is exposed to a neutral pH. pH could influence barrier function through regulation of extracellular lipid processing, because two of the hydrolytic enzymes exhibit acidic pH optima (b-glucocerebrosidase and acid sphingomyelinase), while two others (group 1 secretory phospholipase A2 and steroid sulfatase), display neutral-to-alkaline pH optima. Integrity/cohesion, which is inversely related to desquamation, represents a second possible pH-dependent SC function, regulated by two serine proteases: the SC chymotryptic (SCCE) and tryptic (SCTE) enzymes. Since SCCE and SCTE exhibit neutral-to-alkaline pH optima, they are likely to be active in the lower SC (=stratum compactum), and they probably increase in pathological skin, which is characterized by increased pH. Finally, an acidic SC pH could also restrict cutaneous inflammation. The SC contains precursors of both IL-la and IL-lb, which generate their active pro-inflammatory products in response to a variety of external perturbations also potentially mediated by SCCE. Consistent with this hypothesis, the pH of dermatitic (inflamed) skin of all types is typically about one unit higher than the pH of normal SC. In this proposal, we will explore the pH-dependent processes that regulate all three of these key SC functions. We will use a k.o. mutant, and inhibitor-based models, and combinations thereof, to delineate the role of each pathway in SC acidification. Then, we will explore how such pathway influence the three key functions. The information from these studies could lead to entirely new approaches to the therapy of common skin disorders.